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Reaction of 1,2‐di(tetrazol‐2‐yl)ethane (ebtz) with Fe(BF4)2⋅6 H2O in different nitriles yields one‐dimensional coordination polymers [Fe(ebtz)2(RCN)2](BF4)2⋅nRCN (n=2 for R=CH3 (1) and n=0 for R=C2H5 (2) C3H7 (3), C3H5 (4), CH2Cl (5)) exhibiting spin crossover (SCO). SCO in 1 and 3–5 is complete and occurs above 160 K. In 2, it is shifted to lower temperatures and is accompanied by wide hysteresis (T1/2↓=78 K, T1/2↑=123 K) and proceeds extremely slowly. Isothermal (80 K) time‐resolved single‐crystal X‐ray diffraction studies revealed a complex nature for the HS→LS transition in 2. An initial, slow stage is associated with shrinkage of polymeric chains and with reduction of volume at 77 % (in relation to the difference between cell volumes VHS−VLS) whereas only 16 % of iron(II) ions change spin state. In the second stage, an abrupt SCO occurs, associated with breathing of the crystal lattice along the direction of the Fe–nitrile bonds, while the nitriles reorient. HS→LS switching triggered by light (808 nm) reveals the coupling of spin state and nitrile orientation. The importance of this coupling was confirmed by studies of [Fe(ebtz)2(C2H5CN/C3H7CN)2](BF4)2 mixed crystals (2 a, 2 b), showing a shift of T1/2 to higher values and narrowing of the hysteresis loop concomitant with an increase of the fraction of butyronitrile. This increase reduces the capability of nitrile molecules to reorient. Density functional theory (DFT) studies of models of 1–5 suggest a particular possibility of 2 to adopt a low (140–145°) value of its Fe‐N‐C(propionitrile) angle. [ABSTRACT FROM AUTHOR]

1,4-Di(1-ethyl-1,2,3-triazol-5-yl)butane (bbtre) was prepared by lithiation of 1-ethyl-1,2,3-triazole, followed by alkylation with 1,4-dibromobutane. The ligand bbtre forms a three-dimensional network with Fe(II), [Fe(bbtre)3](ClO4)2·2CH3CN, that exhibits thermally induced spin crossover (SCO). A change of temperature or change of spin state results in various types of structural transformation, leading to different structures that are stable in strictly defined temperature ranges. As a result, there are three spin crossover transitions arranged via two different paths. Thus, cooling below 280 K involves a HT(HS) → LT(HS) (HT, high temperature structure; LT, low temperature structure; HS, high spin) phase transition (PT), which is associated with conformational changes of the bbtre molecules and with deformation of the polymeric skeleton. In the LT phase incomplete and reversible LT(HS) LT(HS/LS) spin crossover occurs (LS, low spin). In contrast, rapid cooling (of a sample not previously thermally treated) allows the HT(HS) → LT(HS) phase transition to be avoided, and so complete HT(HS) → HT1(LS) SCO occurs. This means that the PT plays the role of a switch, which allows a choice of one of two ways in which the SCO will proceed. After rapid cooling, further heating to 150 K and subsequent cooling results in a reversible HT1(HS) HT1(LS) spin crossover (T ↓1/2 = 130 K, T ↑1/2 = 131 K). However, raising the temperature to 170–200 K leads to formation of a modulated structure HT2(HS) exhibiting the next reversible HT2(HS) HT2(LS) SCO (T ↓1/2 = 121 K, T ↑1/2 = 123 K). Finally, heating above 200 K involves the HT2(HS) → LT(HS) PT and results in a LT(HS) structure exhibiting incomplete LT(HS) LT(HS/LS) spin crossover. [ABSTRACT FROM AUTHOR]

In [Fe(ebbtr)2(CH3CN)2](CF3SO3)2·4CH3CN spin crossover is associated with the occurrence of “normal” and “reverse” hysteresis loops separated by a region of stable HS form. This results from trans–trans→gauche–trans conformational changes of ebbtr molecules and anion reorientation, which occur in different ways during cooling and during heating. [ABSTRACT FROM AUTHOR]

The abrupt high spin (HS)→low spin (LS) transition (T↓1/2=136 K) in [Fe(hbtz)2(CH3CN)2](BF4)2 (hbtz=1,6‐di(tetrazol‐2‐yl)hexane) is finished at 100 K and further thermal treatment influences the spin crossover. Subsequent heating involves a change of the spin state in the same way (T↑1/2=136 K) on cooling. In contrast, cooling below 100 K triggers different behavior and T↑1/2 is shifted to 170 K. The extraordinary structural changes that occurred below 100 K are responsible for the observed diversity of properties. A unique feature of the low‐temperature phase is the rebuilding of the anion network expressed by a shift of anions inside the polymeric layer at a distance of 1.2 Å as well as the relative shift of neighboring layers at over 4 Å. These structural alterations, connected with a phase transition, become the origin of the strain, which in most cases causes crystal cleaving. In a sample composed from crystals crushed as a result of the phase transition or as a result of mechanical crumbling, the hysteresis loop vanishes; however, annealing the sample allows to its partial restoration. A replacement of acetonitrile by other nitriles leads to preservation of the polymeric structure and spin crossover, but no phase transition follows. Spin and phase transitions: Two‐dimensional coordination network [Fe(hbtz)2(CH3CN)3](BF4)2 (hbtz=1,6‐di(tetrazol‐2‐yl)hexane) exhibits a high spin (HS)⇄low spin (LS) transition, which is accompanied by significant structural changes, in particular, involving the shift of neighboring polymeric layers at distances above 4 Å. Occurrence of a structural phase transition as well as hysteresis loop is blocked by crystal cleavage, however, sample annealing restores hysteretic behavior (see scheme). [ABSTRACT FROM AUTHOR]

The abrupt high spin (HS)→low spin (LS) transition (T↓1/2=136 K) in [Fe(hbtz)2(CH3CN)2](BF4)2 (hbtz=1,6‐di(tetrazol‐2‐yl)hexane) is finished at 100 K and further thermal treatment influences the spin crossover. Subsequent heating involves a change of the spin state in the same way (T↑1/2=136 K) on cooling. In contrast, cooling below 100 K triggers different behavior and T↑1/2 is shifted to 170 K. The extraordinary structural changes that occurred below 100 K are responsible for the observed diversity of properties. A unique feature of the low‐temperature phase is the rebuilding of the anion network expressed by a shift of anions inside the polymeric layer at a distance of 1.2 Å as well as the relative shift of neighboring layers at over 4 Å. These structural alterations, connected with a phase transition, become the origin of the strain, which in most cases causes crystal cleaving. In a sample composed from crystals crushed as a result of the phase transition or as a result of mechanical crumbling, the hysteresis loop vanishes; however, annealing the sample allows to its partial restoration. A replacement of acetonitrile by other nitriles leads to preservation of the polymeric structure and spin crossover, but no phase transition follows. Spin and phase transitions: Two‐dimensional coordination network [Fe(hbtz)2(CH3CN)3](BF4)2 (hbtz=1,6‐di(tetrazol‐2‐yl)hexane) exhibits a high spin (HS)⇄low spin (LS) transition, which is accompanied by significant structural changes, in particular, involving the shift of neighboring polymeric layers at distances above 4 Å. Occurrence of a structural phase transition as well as hysteresis loop is blocked by crystal cleavage, however, sample annealing restores hysteretic behavior (see scheme). [ABSTRACT FROM AUTHOR]

The complexes [M(3ditz)3]X2 [X = ClO4-, M = FeII (1), ZnII (2); X = BF4-, M = FeII (3), ZnII (4); 3ditz = 1,3-di(tetrazol-1-yl)propane] were prepared. Disordering of the propylene chain in 3ditz is not affected by thermally induced spin crossover in 1 (T1/2↓ = 149 K, T1/2↑ = 150 K) and 3 (T1/2↓ = 158, T1/2↑ = 161 K). Low-spin to high-spin (LS→HS) switching triggered by laser-light irradiation (520 nm) also does not influence the disordering of the ligand, leading to restoration of the initial HS phase. Pressurization of crystals results in a reduction of the Fe-N distances by about 0.19 Å, which indicates practically complete spin crossover (SCO). Also, in this case, the disordering of the ligand remains unchanged, although thermally and pressure-induced SCO is accompanied by different changes of 3ditz molecules. Upon thermally induced SCO, perturbation resulting from the reduction of the Fe-N distances is accompanied by a slight elongation of the bridging ligand. In contrast, in the range 0.25-0.55 GPa, the pressure-induced SCO is associated with shrinkage of the 3ditz molecule. Further elevation of pressure results in the inversion of structural changes and - similar to temperature-induced spin crossover - a slight elongation of the ligand molecule takes place. [ABSTRACT FROM AUTHOR]

The complexes [M(3ditz)3]X2 [X = ClO4-, M = FeII (1), ZnII (2); X = BF4-, M = FeII (3), ZnII (4); 3ditz = 1,3-di(tetrazol-1-yl)propane] were prepared. Disordering of the propylene chain in 3ditz is not affected by thermally induced spin crossover in 1 (T1/2↓ = 149 K, T1/2↑ = 150 K) and 3 (T1/2↓ = 158, T1/2↑ = 161 K). Low-spin to high-spin (LS→HS) switching triggered by laser-light irradiation (520 nm) also does not influence the disordering of the ligand, leading to restoration of the initial HS phase. Pressurization of crystals results in a reduction of the Fe-N distances by about 0.19 Å, which indicates practically complete spin crossover (SCO). Also, in this case, the disordering of the ligand remains unchanged, although thermally and pressure-induced SCO is accompanied by different changes of 3ditz molecules. Upon thermally induced SCO, perturbation resulting from the reduction of the Fe-N distances is accompanied by a slight elongation of the bridging ligand. In contrast, in the range 0.25-0.55 GPa, the pressure-induced SCO is associated with shrinkage of the 3ditz molecule. Further elevation of pressure results in the inversion of structural changes and - similar to temperature-induced spin crossover - a slight elongation of the ligand molecule takes place. [ABSTRACT FROM AUTHOR]

The role of the covalent connections of iron(II) spin-crossover centres in the transmission of perturbation produced by the HS ↔ LS transition has been investigated for the new 3D coordination polymer {[Fe(qbtr)3](ClO4)2}∞ [qbtr = 1,5-di(1,2,3-triazol-1-yl)pentane]. In this complex, a sequence of structural phase transitions occurs. A Ph1 (HS,HS) → Ph2 (HS,HS,HS) transition precedes spin crossover and is associated with an increased number of crystallographically unique iron(II) ions, whereas the change of spin state Ph2 (HS,HS,HS) → Ph3 (HS,LS) triggers another structural phase transition accompanied by a reduction in the number of the unique iron(II) sites. Iron(II) ions occupying two crystallographically unique Fe1 and Fe2 sites form the nodes of a coordination network in the Ph1 phase. The characteristic feature of the polymeric skeleton is the presence of three kinds of ligand bridges tethering the Fe1 ···Fe1, Fe1 ···Fe2 and Fe2 ···Fe2 sites. The ligand linkages differ from each other in rigidity, and the diversification of their properties arises from the susceptibility of the ligand molecules to undergo conformational changes. The high rigidity of the bridge established in the Ph2 phase between iron(II) ions occupying Fe2 sites allows for the propagation of the perturbation produced by the HS (Fe2) → LS (Fe2) transition through the crystal lattice. A strong compression along the Fe2 ···Fe2 bridging direction occurs. In contrast, the ligand molecules that form the two remaining types of bridge are disordered in the Ph2 phase and the change of spin state for the iron(II) ions occupying Fe2 sites induces their ordering, which leads to an increase in the separation between donor atoms. In effect, perturbation cannot be effectively transmitted along the Fe1 ···Fe1 and Fe1 ···Fe2 linkage directions because the 'elongation' of conformationally labile ligand molecules compensates for the shortening of the Fe2-N bond lengths. [ABSTRACT FROM AUTHOR]

Novel tripodal ligand 1,1',l"-tris(tetrazol- I-ylnsethyl)methane (IIItz) and products of its reactions with perchlorate as well as with tetrafluoroborate salts of iron(II) are presented The isostructural complexes, [Fe(IIItz)2](C104)2 and [Fe(IIItv)2](BF4)2, were isolated as two-dimen- I sional (2D) coordination networks revealing a honeycomb-like pattern with cages occupied by disordered anions. IIItz molecules act as a tridentate ligand bridging three adjacent Fe(II) ions, and the nitrogen N4 atom of six tetrazole rings (tz) is placed in octahedron vertices of FeN6 chromophores. The complexes, crystallizing in the P3 space group, were characterized by variable-temperature single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. Variable-temperature magnetic susceptibility measurements show that both systems undergo abrupt and complete spin transition with T↑½ =T↓½ = 176 K for perchlorate and T↑½ = 193.8 and T↓½ = 192.8 K for the tetrafluoroborate analogue. Change of spin state in both complexes is accompanied by a thermochromic effect. The HS→LS transition in [Fe(IIItz)2](C104)2 involves shortening of the Fe-N4 bond lengths from 2.171(2) A (293 K) to 2.002(1) A (100 K). In [Fe(IIItz)2](BF4)2, lowering of temperature from 293 to 100 K is accompanied by shortening of the Fe-N4 distances from 2.179(2) to 1.987(2) A, respectively. Perchlorate in [Fe(IIItz2](C104)2 or tetrafluoroborate anions in [Fe(IIItz)2](BF4)2 are engaged in the formation of intermolecular contacts within as well as with the neighboring 2D layer. [ABSTRACT FROM AUTHOR]

In a 1D network of ¹8{[Fe(CH3CN)4(pyrazine)](ClO4)2}, the presence of four neutral nitrile molecules besides the pyrazine donors in the first coordination sphere of iron(II) allows one to achieve a ligand field strength appropriate for the "spin-crossover" occurrence. [ABSTRACT FROM AUTHOR]